Despite current testing promotion efforts to critically reduce the rate of new HIV infections in the US, men who have sex with men (MSM) may not be aware of the need or motivated to test for HIV/STIs at regular intervals. Current HIV testing guidelines recommend at least annual testing, with more regular testing (every 3 to 6 months) for those with identifiable risk factors, including substance use and having multiple or anonymous sex partners. However, recent studies show that one-third of US MSM have not tested for HIV in the past year, despite the trend of increasing unprotected sex between 2005 and 2011 in this population. To address this gap, we propose a 3-year study to develop and pilot test a tailored, theoretically-based (in the Information- Motivation-Behavioral Skills model) smartphone application (or app) intervention to facilitate regular HIV testing among a diverse nation-wide sample of MSM who are at-risk for HIV. Derived from our extensive HIV prevention research with MSM, core components of the smartphone app intervention will include: a) regular assessment of risk behaviors via personal profile; b) tailored recommendations for frequency of HIV testing based on individual's risk patterns and profile; c) GPS-enabled HIV testing location finder; d) motivational messages addressing critical barriers to testing; and e) a graphically appealing interface tailored to the unique cultural and social preferences of MSM. The study will be conducted in 3 phases with corresponding aims. In Aim 1, 6 focus groups of high-risk MSM will be conducted in 2 US cities (Minneapolis, MN and Detroit, MI) to determine which app features and functions are most likely to support downloading, initiating use, and sustaining engagement over time of the tailored HIV/STI testing app. The smartphone app will be developed in collaboration with our technology partner in Aim 2, and usability tested among 6 members of the target population in Detroit, MI and Minneapolis, MN. Finally, the goal of Aim 3 is to assess the feasibility and acceptability of the smartphone app intervention to promote regular HIV testing at tailored, prescribed intervals among racially and ethnically diverse US MSM. A national sample of 120 at-risk HIV-negative/unknown MSM who have not been tested for HIV in the past year will be randomly assigned to either the experimental (n=60) or control condition (n=60) and followed for 6 months, with assessments at baseline, 3- and 6-months. The control condition will be a non-tailored, information-only version of the HIV testing app. In addition to feasibility and acceptability measures, preliminary effect estimates will be calculated to inform a subsequent R01 trial. This proposed study would help advance science of HIV prevention interventions by harnessing smartphone app technology to promote regular testing among US MSM who at-risk for acquisition of HIV. Findings will inform the development and implementation of a large, national RCT study to test for efficacy. If effective in the subsequent R01 trial, the intervention will be readily available for scale-up for national dissemination by partnering with clinics, community-based organizations, and websites used by MSM in the US.